GROUP IV 181
being used by some large industrial organisations but others use
petroleum from a refinery. The second stage in the process is the
so-called %water-gas shift' reaction; this reaction was originally used
with vwater-gas'—a mixture of CO and H 2 obtained by passing
superheated steam through white hot coke. The gaseous mixture
containing an excess of steam still at 10 atmospheres pressure, is
passed at 700 K over an iron catalyst when the carbon monoxide
reacts with the steam to form carbon dioxide and hydrogen:
CO + H 2 O -> CO 2 + H 2
In one process the carbon dioxide is removed using potassium
carbonate solution, potassium hydrogencarbonate being produced:
K 2 CO 3 + H 2 O + CO 2 -» 2KHCO 3
This reaction can be reversed by heat and the potassium carbonate
and carbon dioxide recovered. (Other compounds which absorb
carbon dioxide and evolve it again at a lower temperature are also
in common usage*).
STRUCTURE
Carbon dioxide has a linear structure. The simple double-bonded
formula, however, does not fully explain the structure since the
measured carbon-oxygen bond lengths are equal but intermediate
between those expected for a double and a triple bond. A more
accurate representation is, therefore, obtained by considering carbon
dioxide as a resonance hybrid of the three structures given below:
O = C^O <-> O=C=O <-> O—C = O
(a) (b) (c)
PROPERTIES
Carbon dioxide is a colourless gas which is virtually odourless and
tasteless. Its density, relative to air, is 1.53; hence it accumulates at
- Some of the carbon monoxide and hydrogen produced in the steam-naphtha
reforming process react to form methane:
CO + 3H 2 <±CH 4 4- H 2 O
This reaction is an undesirable side reaction in the manufacture of hydrogen but
utilised as a means of removing traces of carbon monoxide left at the end of the
second stage reaction. The gases are passed over a nickel catalyst at 450 K when
traces of carbon monoxide form methane. (Methane does not poison the catalyst in
the Haber process -carbon monoxide docs.)
